The invention pertains to a process for the production of castings of a melt of a reactive metal from the group consisting of titanium, titanium alloys, and titanium-based alloys in a reusable casting mold.
There is an increasing demand for structural components of titanium and alloys containing considerable amounts of titanium, because these materials have a low specific gravity and are nevertheless extremely strong, provided that the specific properties of titanium, including its high melting point and its reactivity at high temperatures, are taken into account sufficiently. At its melting temperature, titanium reacts not only with reactive gases, including oxygen in particular, but also with oxides and nearly all types ceramics, because ceramics usually consist at least primarily of oxide compounds. Because of the great affinity of titanium for oxygen, oxygen is taken away from the oxides, which leads to the formation of titanium oxides. Some materials which have been found to give excellent results in certain areas of application are listed below by way of example:
pure titanium, PA1 Ti 6 Al 4 V, PA1 Ti 6 Al 2 Sn 4 Zr 2 Mo, PA1 Ti 5 Al 2.5 Sn, PA1 Ti 15 V 3 Al 3 Cr 3 Sn, PA1 Ti Al 5 Fe 2.5, PA1 50 Ti 46 Al 2 Cr 2 Nb, and PA1 titanium aluminides. PA1 valves for internal combustion machines, PA1 turbine wheels and turbine vanes, PA1 compressor rotors, PA1 biomedical prostheses (implants), and PA1 compressor housings in aircraft construction.
The use of titanium aluminides, e.g., TiAl, as a material for numerous components deserves special mention. Because of their low density, relatively high strength at high temperatures, and corrosion resistance, the titanium aluminides are considered the optimum material for various areas of application. Because these materials are very difficult to work, the only way to shape them is by casting. Especially in conjunction with casting, however, titanium-containing metals present further problems, which will be discussed in greater detail below.
Some examples of the use of titanium-containing materials are listed here:
Especially in the area of motor vehicle racing, both intake and exhaust valves made of certain titanium alloys have been found to yield excellent results. Thought is therefore now being given to the general use of such valves in internal combustion machines of all types.
In the article by Schadlich-Stubenrauch et al. entitled "Numerical simulation of the alpha case as a quality criterion for the investment casting of small, thin-walled titanium parts," published at the Sixth World Conference on Titanium, France 1988, pp. 649-654, the problems which arise when titanium alloys are cast into molds of oxide materials are described. Titanium oxides form on the surface of the casting, but in addition oxygen also goes into solution at the grain boundaries at the rate of as much as 10 wt. %. It is therefore necessary to refinish the surfaces of the casting, which can be done either chemically or by cutting away the material. Of necessity, the thickness of the oxygen-containing surface layer increases with the length of the cooling period, which limits the use of molds of oxide materials for thin-walled workpieces. In addition, it is stated that it is advisable to subject the finished workpieces afterwards to a hot isostatic pressing step (HIP process). As a result, the cost of such components increases quite dramatically. The article studies these relationships on the basis of wedge-shaped castings.
The article by Tsutomu Oka et al. entitled "Manufacturing of automotive engine valves by plasma package melting of scrap titanium," published at the Sixth World Conference on Titanium, France, 1988, pp. 621-626, tells us that the valves used in internal combustion engines can be made of titanium alloys. For the production of the intake valves, which operate at relatively low temperatures of up to about 450.degree. C., the alloy Ti 6 Al 4 V is recommended. For the exhaust valves, the operating temperatures of which can be as high as 700.degree. C. or more, the alloy Ti 6 Al 2 Sn 4 Zr 2 Mo 0.1 Si is recommended, in which case it is pointed out that it is difficult to produce parts with a diameter of less than 10 mm because of the difficulty of machining this material. It is therefore recommended that, for these exhaust valves, the valve plates be made of the latter titanium alloy and that it be combined with valve shafts made of Ti 6 Al 4 V. This article also demonstrates the circuitous routes which must be taken to work around the material properties of certain titanium alloys during processing.
Through the article by Zwicker et al. entitled "Evaluation of centrifugally cast TiAl5Fe2.5 alloy for implant material", it is known that hip joint prostheses or implants can be produced from the titanium alloy cited in the title in a copper mold by means of a centrifugal casting process. It is stated that, as a result of the fast quenching rate attributable to the copper, the advantage of a fine-grained surface is obtained; it is also pointed out, however, that the fast cooling rate leads to the formation of pores caused by gas inclusions and to the formation of shrink holes, which lead to a notch effect. It is therefore recommended that the pores and shrink holes be eliminated by a HIP process, although it is expressly pointed out that, even at a pressure of 1,000 bars, it is possible to close only small pores and shrink holes, not pores at the surface of the workpiece, which are in fact opened even wider by the pressure and which intensify the notch effect even more. To correct these defects, it is stated that the surface irregularities be closed by welding, as a result of which, however, the disadvantage of a coarse grain structure is obtained in return. As the parameters for the HIP process, it is stated that a pressure of 1,000 bars should be allowed to act for 3 hours at 950.degree. C. The article contains the further suggestion that the copper mold must have a relatively high weight in comparison to that of the workpiece to avoid reactions between the liquid titanium alloy and the surface of the copper. This suggestion allows the single conclusion that the copper mold must be used in the cold state and that therefore any preheating of the copper mold must be omitted, which is associated in turn with an undesirably fast quenching rate.
It can be derived from the state of the art sketched above that extremely strict requirements must be imposed on the selection of the mold material, that is, of the material used for the casting mold, and that, in addition, narrowly defined processing guidelines must be followed in order to prevent damage to the workpiece or to the chill form or casting mold. In a sense, therefore, the properties of the melt and those of the casting mold are diametrically opposed, and it should also be remembered that most titanium alloys must be cast at temperatures which are significantly above 1,500.degree. C., whereas copper has a melting point of 1,084.degree. C., and the eutectic point of the alloy copper/titanium is 865.degree. C.
U.S. Pat. No. 5,119,865 deals with the problem of improving the dimensional accuracy or accuracy of shape of centrifugal casting molds of copper and the ease with which workpieces of titanium alloys can be removed from the mold. According to this patent, zirconium, chromium, beryllium, cobalt, and silver are added to the copper as alloying elements, the sum of all the alloying elements not exceeding 3 wt. %. A comparison example, in which the copper was alloyed with 18 wt. % of nickel, did not lead to success. The patent deals with the electrical conductivity of the material, not with its thermal conductivity, so that the problems of a fast quenching rate and of the formation of pores and shrink holes were not discussed. On the other hand, this patent does discuss the disadvantages of ceramic or oxide mold materials.
From the articles by:
1. Krone: "Production and properties of precision and compact castings of titanium materials", published in Giesserei 65, No. 20, pp. 540-549, Sep. 28, 1978; and
2. Krone et al.: Titanium castings: manufacture and properties," published in AFS International Cast Metals Journal, Vol. 2, No. 1, pp. 37-40, March 1977, we know that precision casting molds can be produced with a "metallic front layer", although these molds are so-called "lost" molds, used according to the lost wax method for a single use. Nonmetallic materials, primarily oxides, are present behind the metallic front layer. The metallic front layer is again made from a paste (slurry) and fired, this paste consisting of a mixture of metal powders, including tungsten, tantalum, niobium, and/or molybdenum powders together with inhibitor formers and liquid organometal compounds. This layer therefore contains significant amounts of nonmetallic constituents. The entire mold is then supposed to be removed by water jets, airless blast cleaning, sandblasting, etc., from the castings, which it is still necessary to clean afterwards by the use of salt baths and manual treatment. This works only because the front layer does not have a high degree of cohesiveness in itself as a result of the presence of nonmetallic materials in it.
A disadvantage is that the front layer cannot prevent the uptake of oxygen into the castings, because, first, the front layer itself contains oxygen compounds; second, because it is permeable to oxygen coming from the ceramic mass behind it; and, third, because in particular it reaches a very high temperature especially during the casting operation, which promotes the migration of oxygen.